Careful. Legal ramifications could result in any sort of physical assault. Did you slap/hit/punch/any other way phys. contact the person, first? How angry are you? Are you mad enough to pursue legal action against this person? ANY time someone puts their hands on you without your invitation to do so, you should call the police, and file a report. Were there any witnesses to this altercation? etc. If you seek out to slap them back...or worse, you could end up in jail. Yes. Even if you feel justified in your retaliation. Until rules are changed, that is generally the way things are.
First, what did you say or do to get slapped and receive that comment? I am not saying slapping you is right, but no one just walks up out of no where and does this.
Second, Don't hit back that just brings you down to their level. You could make a comment about already having a life and it is one that does not include violence. Also from there it would depend on what started being slapped in the first place.
slap him back and say either 1."smack me once i'll smack you back smack me twice it brains you lack." or 2. breathe in deep and lift your chin "smack me again and wont even have the life your living right now" then say in sweet voice "tootalo" and skip away